Extrapolated It From Probabilities
by Td03
Summary: How Satomi Rentarou, an ordinary boy, ended up on the front lines of the Gastrea War. Slight Canon Divergence.


**Extrapolated It From Probabilities**

 **Disclaimer: _Black Bullet_ is owned by Shiden Kanzaki, Sentai Filmworks, Kinema Citrus, Orange, Geneon Universal Entertainment, AT-X, Showgate, NBCUniversal,** and **Entertainment** **Japan.**

 **Summary: How a boy, an ordinary boy, ended up on the front lines of the Gastrea War.**

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 _ **S** u **n** ' **s** d **y** i **n** g_

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He was just a kid who had taken to the streets. His parents were relatively rich, but he didn't like being shoved aside for babysitters and tutors.

He wondered where they had gone, and why he ended up here.

Beep Beep Beep Beep Beep Beep Beep Beep Beep

Satomi abruptly sat up, and pain exploded behind his eyes. He took small, consistent breaths to calm down.

He knew about hospitals. He just had never landed himself at one. He figured he'd be around when he's thirty or so, not six.

Strange. There were black metal scraps on the other side of the bed. Wires. Tubes. Body models. Had doctors been operating?

 _Good eye coordination._

Satomi lifted his right hand automatically — and did it feel a little heavier somehow? — and touched his left eye softly. It could be that he was hearing things, but he was sure the two made a sound like metal touching one another.

He needed to see a mirror. With that in mind, Satomi lifted his legs — what had happened? His right leg and left leg feels different! — and got of the bed. He wobbled for a good moment before getting a sense of balance and walked towards the middle of the room. He tried to look for a mirror but there was none. Though he did spot a door, maybe it's a bathroom door.

Then his ears picked up a doorknob turning and he turned to see who had come in. The person was wearing a lab coat and clean set of surgery gloves. The doctor was a woman, with baggy eyes and crooked smile Satomi remembered hazily.

They looked at each other for a good while. Then the doctor opened her mouth. Satomi registered her lip movements before the syllables she was saying. "Did you walk all by yourself?" Satomi tilted his head. She was talking… normally. But she looked slow, somehow. As if in a constant slow motion.

"Rentarou-kun?" He read her lips _before_ picking up her voice.

"Yes…" he said uncertainly. It sounded like a question. He repeated, "Yes."

Her lips twitched up, and Satomi figured she was smirking before she was smirking. "You're seeing me as if I'm being slow, aren't you?" She didn't give him time to reply before walking in and sitting on a chair near the bed. She asked him to sit too, so Satomi did. He figured he probably shouldn't, though why, he didn't know; she's a _doctor_.

The doctor didn't seem like a friendly person. But her voice sounds friendly, and she was good in explaining things.

Satomi got the gist that he had — in the process of saving someone during the Gastrea _Giant-Monster-insect-fire-fire-fire-smoke_ attack — he had lost his entire right leg, right arm, and his left eye. The person Satomi had saved, a girl named Kisara Tendo, had begged her to save his life, and at the time the only one available for a surgery was her, Dr. Sumire.

It wasn't a legal procedure, the woman stressed. Satomi didn't sign up for anything. Satomi was just fighting to stay conscious. He was only fighting for life against the damage on his small body. She told him about Varanium, the metal ore she had used to create his prosthetic limbs and computer eye. The varanium metal wasn't strictly hers, it was metal recently discovered to be able to drive off and strong enough to harm a Gastrea. She told him that his surgery itself was an agreement of his consent to be part of a Mechanized Soldier Unit.

She said she was sorry for him. Satomi wasn't sure how he feels about being a _soldier_ so young, so he switched the topic and asked how many days had he slept in. She said it's been eight days, and his stomach rumbled in hunger.

They went to the hospital cafeteria to get a meal. He asked for his parents, since the doctor knew his name. Dr. Sumire said he was in the records, but she didn't see hide or hair of his parents. Calling their numbers in the records didn't alert them. She figured either his parents or both his parents' phones are dead.

Days passed by. Satomi was in and out of the hospital, walking around too far for his own good and running back to the protected hospital whenever his computer eye spotted a Gastrea. He didn't know if the monster insects can pick up a human scent. He asked about it and Dr. Sumire had a surprising collection of books about insects in her office.

She also spent a lot of time putting him in tests. Conditioning trials she called them, to see if he's adapting to the metal limbs. She said he was probably born to be a half-man half-robot with how good his coordination and fluid his movements are, much better than the others. Her slip ended up with him pestering her about this 'others' and she told him about the other soldiers.

Unlike him, however, these were military men who had consented for their organs to be replaced with Varanium. She and four other doctors — being the only ones who can dothe surgery — attended to them. Unlike Satomi, they had replaced almost their entire body, only leaving their heart and eyes. They had to adjust to be unable to eat normal food, instead taking special pills one of those four doctors made that will distribute the required nutrients their human body parts needed. They have horrible coordinations with it, unlike the boy. But Satomi is also the only one amongst the seven mechanized soldiers with no fighting experience. All he had were small acrobatics he learned from street kids, and nothing about figuring out how to punch and block and parry.

He wondered aloud if he will ever see them. She just shrugged, and told him he better hope not to get called on. On what she didn't say.

After a week, he met Kisara Tendo, the girl he had saved back at the first Gastrea attack. She was insistent where he was quiet, and by childish logic they became friends pretty quickly. It turns out that the Tendo family had been the one who had been footing his bill, and that they were going to adopt him because it turns out his parents had died.

The week had felt more like a dream. He didn't really mourn. But he did felt — a detached sort of — happiness, knowing he's having a family that has someone who will be a sister that wants to spend time with him.

Six months after the surgery, he turned seven. And that was when he was called in by the government to fight the front lines as a key soldier to finish off the Gastrea that ordinary soldiers had failed to subdue. The government were trying to build a monolith of varanium metals around Tokyo, but hadn't managed past a hundred meters yet, and Gastrea were still raiding some parts of the city.

He wondered what the soldiers thought. Looking at a kid wearing a uniform and helmet and no guns, kicking the dirt with unbelievable strength to jump and finish off giant insects which armors could shrug off ordinary bullets. His small body gave him the nimbleness and agility that let him able to avoid Gastrea attacks, their poison stings and web shots. His computer eye allowed him to see the world in slow-motion, and he can react accordingly to avoid all offense.

His operations were a series of successes. Soldiers cheered whenever a Gastrea fell. Off of the battlefield though, they protested. The public have no idea of the unit, only the military; and said military criticized the wisdom of letting a child soldier go alone, even knowing the fact that having an ordinary partner alongside would only hinder things.

It left him to ask why he couldn't partner up with the other mechanized soldiers. He found that the seven first generations were sent on different parts of the area. Three were dead already. Varanium supply were restricted for another operation because it was all going to be used to make the monoliths, and the small amount that they set aside were going to be used to make bullets. Mechanized soldiers weren't a guarantee program they want to afford wasting the precious metal.

Sumire though, he found out why she said his operation was not strictly legal. It wasn't because he didn't consent. The varanium alloy she used is one she had stolen from a mine she had found were being actively avoided by Gastreas. She had them brought all those metals to her house, warehouse, labs, and bought an amount legally to cover up why she even had them in the first place.

Amongst the parties he attended as a 'normal' child adopted by the Tendo family, amongst the front lines he participated as a trump card, amongst the visits to Dr. Sumire's office to adjust his metal limbs as he grew, he was given psychological examinations. He wasn't supposed to know the results, but Sumire was loose lipped about it. Conclusion of the summaries were that he was too perfect on the front lines, and they worry as to how he will ever adjust to normal life later on, after the monoliths are finished.

He made a point by learning how to cook, clean, pretty much every skill he would need to take care of a house whenever he wasn't spending his free time studying insects or playing with Kisara.

Three years passed. Out of the seven mechanized soldiers, five were killed, one went MIA, while Satomi remained. Two of the four professors (the four people who were genius enough to be able to do the operations) were killed. Sumire remained in Japan. The other one was stuck in Germany, unable to produce another mechanized soldier because he no longer have any Varanium ore, and countries have stopped working together after the first year of Gastrea attack.

The monoliths were all built though, and Satomi welcomed the call for him to fall back.

Sumire adjusted Satomi's limbs as his human body grows, and Satomi was learning about how to do it himself too: "In case you died in an untimely manner, Sensei."

One day, a three year old girl was found to be able to do 'supernatural' things, and destroyed a _lot_ of public property. More occurrences popped up (leaving property damage and dead victims) and it was discovered that all of them were caused by girls with shiny red eyes. All the girls found with these characteristics were then taken to research facilities. Their potential was immediately recognized.

But they were all three year old girls, still dumb and clueless.

Adults trained and tested them, kept and captured them, forced them to learn how to read, write and take commands to obey. When the first girls they got were now 8 years old, one showed promise in swordsmanship and was taken in by a general to be his partner. The two showed promise in the frontlines and inspired the program of Promoters and Initiators.

The Initiators showed more promise than Mechanized Soldiers who costs too much Varanium. Satomi thought he'd like to have a partner too, so he wouldn't be all alone when he had to fight Gastreas that formed from infected people inside Tokyo. His apartment was too silent for him too.

He had thought the program was something, well, good. But his opinion was choked when his number was called and he entered a room and was introduced with _his tool_ , they said.

She had long, fiery red hair. Slightly tanned skin with callouses and bruises he recognized were built from training with guns and all kinds of weapons. She looked rabid and she _acted_ like a rabid animal. It was only because of his metal limbs he didn't flinch when she bit his right wrist hard. The prostethic skin functioned like a real skin, and didn't tear away from the bite. She let go and dropped her head, as if accepting her fate to be _his tool_.

He dragged her to his home. Introduced himself. Introduced herself to himself. Made a healthy meal and they shared a quiet dinner.

After a week, dinner had become chaotic. He regretted letting her reign the TV, some of the stupid shows must have brainwashed her to think that he was some kind of dream boy with a red string tied to hers. She nearly kissed him when she borrowed his dictionary and found what the word _partner_ means.

It was hassle. Living with a girl ten years younger than you trying to make you do adult stuff to her _every day_ with the same intensity as if she doesn't fail on the task _every day_.

But it was fun.

Sensei and Kisara calls him a pervert, annoyingly. But he loved waking up every day and actually looking forward for life.

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 _S **u** n **'** s **b** o **r** n_

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